The present invention relates to seals between generator stator bars and their end fitting connections, and to a method for sealing a stator bar end fitting to prevent or reduce corrosion due to coolant water flowing through the fitting and the stator bar.
Water-cooled stator bars for electrical generators are comprised of a plurality of small rectangular solid and hollow copper strands brazed to one another to form a bar. The ends of the strands are brazed to an end fitting, typically referred to as stator bar clip. The end fitting serves as both an electrical and a cooling flow connection for the stator bar.
The hollow end fitting typically includes an enclosed chamber for ingress or egress of stator bar cooling liquid, typically deionized water. At one end, the end fitting receives the ends of the strands of the stator bar. The fitting and the peripherally outermost copper strands of the stator bar are brazed to one another. The opposite end of the fitting is connected to a stator cooling conduit.
Liquid cooled stator bar clips have gone through design changes over the years. However, all of them contain solid strands brazed to hollow strands. During operation, the hollow strands carry water to cool off the bar. Water can corrode the copper alloy. Also, over time, leaks can develop about the connection between the stator bar ends and the stator bar fitting as well as between adjacent strands. It is believed that the leak mechanism is due to a two-part corrosion process which initiates in the braze alloy at the interior surface of the braze joint. Stagnant water in the chamber of the fitting is in contact with the braze alloy and the copper strands. This coolant contact with the braze joint and copper strands is believed to cause corrosion and consequent leakage.
Field repair of coolant leaks through the stator bar end connections has been moderately successful. A leak site is identified by external visual examination of where the strands enter the end fitting. External visual leak detection indirectly identifies the leak site because the visible external leak on the outside of the bar and/or fitting may be a considerable distance away from a point within the fitting where the leak begins. This is particularly true if the leak is occurring between the strands of the stator bar.
An epoxy barrier coating method has been used as a leak repair and prevention method. An example of an epoxy barrier coating method is disclosed in U.S. Pat. No. 5,605,590, the disclosure of which is incorporated herein by this reference. This epoxy barrier coating has been applied to provide protection against water initiated corrosion mechanisms along the brazed length of the strand package. Epoxy coating is manually injected. As a result, the process lends itself to human error. If the coating is applied to thick, it can crack. Moreover, visibility is impaired when looking into the clip of a stator bar. The voids and air pockets are possible during injection. In addition, plugged cooling strands may possibly result. Thus, the process is labor intensive and requires 100% inspection. Moreover, the epoxy is challenged by the extreme temperature conditions during assembly, transportation and operational faulted condition. In this regard, during faulted condition, the temperature of the cooling water applied to the exterior of the bar may change rapidly which applies significant thermal stress to the coating. Also, it is difficult with certain clip designs to achieve visual and injection access. As a result, the process can be labor intensive, takes a long time to complete, and produces a high rate of defects.
There is a need for an improved barrier coating for stator bar clips. In particular there is a need for corrosion protection between the stator bars and their clips. The corrosion protection should be robust and be applicable to various stator bar clip designs, including clips for raised hollow strand stator designs.